White-eyed Vireo Vireo griseus
The White-eyed Vireo breeds in the southeastern United States and northeastern Mexico and is only a rare vagrant to California. Like some other birds whose ranges center on the Southeast, it irrupted in 1992: that one year accounted for 12 of the 44 records for California and two of the five for San Diego County.
Migration: Three quarters of California’s White-eyed Vireos have occurred in May and June, but in San Diego only the first record, of one at Point Loma (S7) 7 June 1982 (R. E. Webster, Morlan 1985), fits into this category. Two records, both also from Point Loma, are for fall: one 16–30 October 1988 (J. O’Brien, Pyle and McCaskie 1992), the other 25 October 1992 (P. A. Ginsburg; Heindel and Patten 1996). Most interesting, two records are of singing males remaining for several days in July, in population centers for Bell’s Vireo in riparian woodland: one along the Santa Margarita River near De Luz Creek (D6) 12–17 July 1992 (P. A. Ginsburg, Heindel and Patten 1996), the other along the Sweetwater River between Sweetwater Reservoir and Highway 94 (S13) 5–15 July 2000 (P. Famolaro, McKee and Erickson 2002).
Bell's Vireo Vireo bellii
Early in the 20th century, California’s subspecies of Bell’s Vireo was abundant. Then clearing of its riparian woodland habitat and parasitism by the invading Brown-headed Cowbird decimated it. By the early 1980s the population in the United States was down to about 300 pairs—about half in San Diego County. As a result, the California Department of Fish and Game listed the subspecies as endangered in 1984, and the U.S. Fish and Wildlife Service followed suit in 1986. Listing opened the door to protection of the vireo’s habitat and widespread trapping of cowbirds, leading to a remarkable recovery: an increase by a factor of six in just 15 years (Kus 2002). Nevertheless, many threats remain; weaning the vireo from cowbird trapping in perpetuity will be a delicate experiment.
Breeding distribution: Riparian woodland supporting the Least Bell’s Vireo typically has both a dense canopy, where the birds forage, and a dense understory, where they nest. The population is concentrated in the coastal lowland, especially along the Santa Margarita River, other creeks in Camp Pendleton, along the San Luis Rey River upstream to Pala (D11), and along Windmill and Pilgrim creeks, tributaries of the San Luis Rey. This area accounted for about 74% of the 1423 territorial males known in the county in 1996—and about 59% of California’s total population, demonstrating that it is the core habitat for the entire subspecies (USFWS 1998). By 1998, the population in Camp Pendleton alone had increased to 1010 territorial males, though it dropped to 783 in 2000 (J. and J. Griffith, data courtesy of Camp Pendleton), and the birds spread along small side creeks as well as all the major ones. In the late 1990s, the vireos continued to recolonize sites in northwestern San Diego County where they had not been recorded up until 1996 (USFWS 1998), sometimes in fair numbers, as along Buena Vista Creek (H6; 15 territorial males in 1997, Kus and Beck 1998), Agua Hedionda Creek (I6; seven on 25 April 1999, P. A. Ginsburg), and in a side canyon north of San Marcos Creek at La Costa (J7; seven on 24 June 1998, M. Baumgartel).
Elsewhere in the coastal lowland, the major sites are the San Dieguito River from Lake Hodges east to San Pasqual (K11/K12/J12/J13; 104 territorial males in 1997, Kus and Beck 1998), the San Diego River from Interstate 805 to Santee (Q10/Q11/P11/P12; 55 territorial males in 1997, Kus and Beck 1998), the Sweetwater River from Sweetwater Reservoir to the Rancho San Diego/Cottonwood golf course (S13/R13/R14; 102 territorial males in 2001, P. Famolaro), Jamul and Dulzura creeks (U14/U15/T15; 24 territorial males in 1996, USFWS 1998), Otay River (V11/V12; about 19 territorial males in 1997, Kus and Beck 1998, C. W. Bouscaren), and the Tijuana River valley (V10/V11/W10/W11; 134 territorial males in 1997, Wells and Turnbull 1998). Spread away from the major centers has been less extensive in central and southern San Diego County than in the northwest, though it is still noticeable, with up to three territorial males in La Jolla Valley (L10) 7 May 2000 (K. J. Winter) and three in Sycamore Canyon (O12) 3 May 1998 (I. S. Quon).
In San Diego County’s foothills, Bell’s Vireo is scattered in small numbers at only a few sites, principally the San Diego River above El Capitan Reservoir (L17/M17; five territorial males in 1997, Kus and Beck 1998; N17; four, including a fledgling, 20 June 2000, D. C. Seals), Cottonwood Creek in Hauser Canyon (T20/T21; up to 11 territorial males and eight nesting pairs in 1998, J. M. Wells), and Cottonwood and Tecate creeks in Marron Valley (V17; eight on 12 June 2000, P. P. Beck). At most other foothill sites the species is irregular. For example, it was absent 1997–2002 from a section of Pine Valley Creek (R19) where six to eight territorial males persisted from the late 1980s to 1994 (Winter and McKelvey 1999, Kus et al. 2003b). Along Santa Ysabel Creek at Black Canyon (I16) Bell’s Vireos occurred regularly from the early 1990s to 1998 (pair at nest 22 May, K. J. Winter) but were absent in 2002 (Kus et al. 2003b). From 1997 to 2003, during his intensive study of the Willow Flycatchers nesting along the San Luis Rey River below Lake Henshaw (F16/G16), W. E. Haas encountered only a single Bell’s Vireo, 31 May 1999. Two observations during the atlas period 1997–2001 well away from previously reported sites were along Boulder Creek at Boulder Creek Road (M19; one singing male and one juvenile 29 June 1997. C. Jones) and along Buena Vista and San Ysidro creeks 2.1–2.4 miles east of Warner Ranch (G19; two singing males 25 June 2000, P. Unitt). At 3800 feet, one in Noble Canyon (O22) 10 and 26 July 2002 was at the highest elevation yet reported for Bell’s Vireo in San Diego County (Kus et al. 2003b); the species was absent at that site during repeated surveys 1992–99.
Oases in the Anza–Borrego Desert also contribute significantly to the population. The vireos there use thickets of mesquite as well as woodland dominated by willows. In this region the important sites are along Coyote Creek, at both Middle Willows (C22; up to five territorial males 28 May 1998, P. D. Jorgensen) and Lower Willows (D23; 18 territorial males or pairs in 2000, Wells and Kus 2001; 31 in 2002, J. R. Barth), Borrego Palm Canyon (F23; up to seven territorial males 5–8 July 2001, L. J. Hargrove), along San Felipe Creek near Paroli Spring (I21; up to six singing males 16 June 2000, J. O. Zimmer), near Scissors Crossing (J22; about 17 territorial males or pairs in 2002, 20 in 2003, J. R. Barth), and in Sentenac Ciénaga and Canyon (J23; up to 17 territorial males 9 May 2001, R. Thériault; I23; up to five on 23 April 1997, P. K. Nelson), and along Vallecito Creek, near Campbell Grade (M23/M24; up to 17 territorial males 6 May 1998, R. Thériault) and near Vallecito Stage Station (M24/M25; counts of territorial males varying from 19 in 2002 to in 33 in 1996, M. C. Jorgensen, P. D. Jorgensen; Wells and Kus 2001).
Several smaller oases also support a few Bell’s Vireos. Some of these were previously known: Sheep and Indian canyons (D22), Hellhole Canyon (G23), Yaqui Well (I24), Agua Caliente Springs (M26), and Carrizo Wash and Marsh (O28/O29). Our numbers at Agua Caliente were notably higher than previously reported, up to six territorial males 6 June 1998 (E. C. Hall). Other sites came to light as a result of field work for this atlas and may represent newly established territories: the Borrego Valley’s mesquite bosque (G25), with one territorial male 27 April and 4 June 1998 and an apparent family group of four on 11 June 1998 (R. Thériault), Bow Willow Canyon (P26), with up to four territorial males 12 May 2000 and 19 May 2001, Jacumba Jim Canyon, elevation 1350 feet (Q27), with one on 13 May 2000, and Carrizo Canyon, elevation 1110 feet (R27), with one on 23 April 2000 (L. J. Hargrove). An unexpected site for nesting Bell’s Vireos in the Borrego Valley was Ellis Farms, a commercial nursery (E24); three there 11 June 2001 included a singing male and a fledgling (P. D. Jorgensen).
Although Bell’s Vireo is a characteristically riparian species, it uses upland scrub adjacent to riparian woodland regularly, foraging at distances up to 200 feet from the riparian edge and even nesting in the nonriparian habitat (Kus and Miner 1989). The use of such marginal habitats increases when, after an unusually wet winter, nearby riparian woodland is flooded and the upland habitat becomes unusually lush. In the wet El Niño year 1998, 8 of 31 territories along Pilgrim Creek (F6) were at the base of slopes in mustard that had grown to a height of 10 feet (B. E. Kus). That year, near Sweetwater Reservoir (S12), one pair nested in sage scrub 1 mile from riparian woodland and three others nested in a field dominated by mustard and exotic trees (P. Famolaro). The vireos persisted in these nonriparian territories near Sweetwater Reservoir for some years but eventually abandoned them.
Nesting: Many studies have addressed the Least Bell’s Vireo’s nesting (e.g., Franzreb 1987, Greaves 1987, Kus 1999, 2002). Typically, the birds nest at openings and edges where there is dense vegetation near the ground, placing the nest, on average, about 1 meter off the ground in a fork of slender twigs. On the coastal slope, willows and mulefat predominate as nest sites (USFWS 1998); in the Anza–Borrego Desert, willows and mesquite predominate (Wells and Kus 2001). Even where cowbirds have been trapped almost to elimination, Bell’s Vireos lose many nests to predation; of 25 nests videotaped by Peterson (2002, Peterson et al. 2004), 12 suffered predation, eight to Scrub-jays and three to nonnative scavengers, two to Virginia opossums and one to Argentine ants. Along the Sweetwater River P. Famolaro has repeatedly observed nests destroyed by Argentine ants.
In San Diego County, Bell’s Vireo’s nesting season generally lasts from April to July, with egg laying from about 1 April to late June, rarely mid July. Around Sweetwater Reservoir, P. Famolaro has noted nests with eggs from 4 April to 8 July, nests with nestlings from 21 April to 21 July. On the basis of 34 nests followed in the Anza–Borrego Desert in 2000, Wells and Kus (2001) estimated egg laying to have taken place from 14 April to 16 June, peaking in late April. Our dates for fledglings ranged from 27 April to 21 August.
Migration: The Least Bell’s Vireo usually arrives in San Diego County in the third week of March. During the atlas period our first dates varied from 13 March (1997) and 14 March (1998) to 29 March (2000) and 31 March (2001); arrival was notably late in the two latter years. Fall departure generally takes place from mid August to late September; stragglers in breeding habitat as late as October are rare. At the upper end of Sweetwater Reservoir, surveyed regularly, the latest date on record is 16 September 1998 (P. Famolaro).
Many Least Bell’s Vireos have been banded, and these studies show the birds are highly site tenacious, usually returning in successive years to the same drainage basin, males nearly always to the same territory. Males generally maintain the same territory through a season; females sometimes move from male to male with successive nest attempts (Greaves 1987). But longer-distance dispersal is known on the basis of birds banded in San Diego County observed in Santa Barbara and Ventura counties and vice versa (Greaves and Labinger 1997, USFWS 1998). Among the more notable examples are of an adult female banded on the Santa Clara River that later nested along the Sweetwater River (P. Famolaro) and a young banded along the Santa Margarita River in summer 1987 that was seen in Carpenteria, Santa Barbara County, 24 August–1 September that same year (AB 42:138, 1988).
Sightings of migrant Bell’s Vireos away from breeding habitat are rare, suggesting that most birds fly nonstop between nesting sites in southern California and their next stop in Mexico. In spring such migrants are most likely in the Anza–Borrego Desert, where records during the atlas period were of one in Borrego Springs (G24) 24 and 29 April 2000 (R. Thériault) and one in Fish Creek Wash (L27) 13 April 2000 (M. B. Mulrooney). Fall migrants have been noted at Point Loma (S7) 10 October 1988 (R. E. Webster, AB 43:169, 1989) and 16 October 1993 (G. McCaskie, AB 48:153, 1994).
Winter: Bell’s Vireo essentially vacates the United States for the winter, but 13 winter occurrences are known for San Diego County. Eleven of these are near the coast. Unitt (1984) listed the first five; subsequently, winter records have been published from Carlsbad (I6) 2 February 1982 (E. Copper, AB 36:332, 1982), Coronado (S9) 19 January–3 March 1985 (D. R. Willick, AB 39:211, 1985) and 15 December 2001 (R. E. Webster, NAB 56:224, 2002), and the Tijuana River valley 27 January 1982 (C. G. Edwards, AB 36:332, 1982), 2 December 1990–5 January 1991, and 15 December 1990 (R. E. Webster, G. McCaskie, AB 45:322, 1991). Two winter records are from the Anza–Borrego Desert, of one at Yaqui Well 20 January 1984 (B. Wagner, AB 38:358, 1984) and one in the mesquite bosque 3.3 miles southeast of Borrego Springs 24 January 1984 (SDNHM 42925).
Conservation: Though Stephens (1919a) called the Least Bell’s Vireo “common” in San Diego County, surveys of the best remaining habitat from 1978 to 1981 (Goldwasser et al. 1980, L. R. Salata) revealed only 61 territorial males. More thorough surveys in 1985 raised this to 223—76% of the population in the entire state (Franzreb 1987). Once the subspecies was finally listed as endangered by the California Department of Fish and Game in 1984 and the U.S. Fish and Wildlife Service in 1986, the regulatory mechanism allowing the species’ recovery was in place. The vireo’s subsequent history must be regarded as one of the greatest successes for the Endangered Species Act anywhere in the United States.
Arresting and reversing the loss of riparian woodland was critical to arresting and reversing the vireo’s decline. Once the vireo was formally designated as endangered, section 404 of the Clean Water Act of 1977 obliged the U.S. Army Corps of Engineers and U.S. Fish and Wildlife Service to confer on any proposal to disrupt wetlands—and therefore Least Bell’s Vireo habitat. As a result, the pace of the vireo’s habitat loss, through the installation of reservoirs and the building of roads, housing, golf courses, and other commercial developments, slowed considerably. Designation in 1994 of about half of the habitat occupied by the vireo as “critical’ under the Endangered Species Act helped as well. Once disturbance of many stands of riparian woodland was minimized, the vireo’s habitat was able to spread through natural regeneration. Though noticeable at many places in San Diego County, this spread was most striking in the Tijuana River valley, where riparian woodland increased from almost none in the 1970s to extensive by the late 1990s and vireos increased from one territory in 1980 to 134 by 1997.
Bell’s Vireos recolonize and nest successfully in adequately restored riparian woodland, more rapidly if the restored habitat is adjacent to mature habitat. They nested in restored habitat the first year in Mission Trails Regional Park, where previously occupied habitat was adjacent (Kus 1998), but took eight years to colonize revegetated shores of the San Diego River in Mission Valley, at a site surrounded by commercial development (P. Unitt).
Brood-parasitism by the Brown-headed Cowbird has also been critical to the vireo’s decline. Soon after the cowbird invaded coastal southern California in the early 1900s, the vireo became a primary host (Hanna 1928). In the early 1980s, parasitism rates in San Diego County varied from 47% to 80%; few if any vireos fledge from parasitized nests (USFWS 1998). Even pairs that desert parasitized nests suffer significantly reduced success, as their subsequent nests are parasitized disproportionately often (Kus 1999, 2002).
Once trapping of cowbirds was instituted widely in the late 1980s, the rate of parasitism dropped, to 1% or less in intensively trapped Camp Pendleton (USFWS 1998). Kus (1999, 2002) noted an inverse association between the intensity of trapping and parasitism rate along the San Luis Rey River. It appears that cowbird trapping is most effective where many traps can be deployed and the cowbird population depressed over a wide region. The critical role of trapping in enabling the vireo’s recovery is especially clear in Anza–Borrego Desert State Park, where the habitat has changed little while the vireo population has rebounded. In the Cleveland National Forest, however, where the vireos are scattered and rugged topography makes the traps difficult to deploy and monitor, trapping proved ineffective, parasitism rates remained high, and the vireo deserted two sites in the 1990s (Winter and McKelvey 1999).
Cowbird trapping, however successful, is a finger-in-the-dike approach to managing an endangered species. Ideally, habitat should be managed so as to be less attractive to cowbirds and the parasitism rate kept down to a level where the vireo (and other parasitism-sensitive species) can maintain themselves. Ideally, exposure to some level of parasitism would allow the vireo to persist while compelling it to evolve better defenses, if only an increased rate of deserting parasitized nests (Kus 2002). The cycle of flooding and regeneration has been broken by the dams built on most of San Diego County’s rivers, allowing some riparian woodland to become senescent. Invasion of exotic plants, especially tamarisk and giant reed, threatens native riparian woodland. Some of the vireo’s primary nest predators, especially the Western Scrub-Jay, are on the increase. In spite of the short-term success in recovering the Least Bell’s Vireo, balancing conflicts until the vireo becomes self sustaining is a long-term challenge.
Taxonomy: The Least, V. b. pusillus Coues, 1866, is the drabbest of the four subspecies of Bell’s Vireo; it has only a hint of olive color on the rump in fresh plumage, and that fades to gray by the time the birds return to their breeding range in spring. Identified by Phillips (1991), the January specimen from the Borrego Valley is V. b. arizonae Ridgway, 1903; it has the lower back and rump distinctly olive and the flanks vaguely yellowish. A Bell’s Vireo at Point Loma 10 October 1988 was "felt to be of the nominate race" (R. E. Webster, AB 43:169–170, 1989). This subspecies, breeding in the Mississippi basin and the green extreme of the species, is likely in California as a rare vagrant but has not been confirmed with a specimen or photograph.
Gray Vireo Vireo vicinior
The Gray Vireo is the rarest breeding bird of San Diego County’s chaparral. Its preferred habitat is on south-facing slopes, where the chaparral is dry but dense, mainly between 3000 and 5000 feet elevation. The reason for the Gray Vireo’s rarity is unclear, but susceptibility to cowbird parasitism may confine it to large tracts of chaparral remote from open areas where cowbirds forage. The California Department of Fish and Game has recognized the Gray Vireo as a highest-priority species of special concern. Field work for this atlas led to the discovery of Gray Vireos wintering in the Anza–Borrego Desert, in California’s single largest stand of the elephant tree.
Breeding distribution: The Gray Vireo is concentrated in two regions of San Diego County. The more northern lies in a region of chaparral dominated by chamise and redshank north of Warner Springs and largely east of Highway 79. The largest numbers have been seen along Lost Valley Road south of Indian Flats Campground and along the Pacific Crest Trail east of this road (E19; up to eight, including seven singing males, 27 June 2001, K. L. Weaver). A few birds are scattered west to Aguanga Ridge east of High Point, Palomar Mountain (D15; three singing males 14 May 1999, K. L. Weaver) and east to the south fork of Alder Canyon (C20; four singing males 3 June 2001, L. J. Hargrove).
The southern population is centered in chaparral dominated by chamise and cupleaf ceanothus south of the Laguna Mountains and north of Interstate 8. Here maximum counts from 1997 to 2001 were near Buckman Springs, on slopes both east and west of Cottonwood Valley (R22; 12, including 11 singing males, 15 May 1999, L. J. Hargrove), along the Pacific Crest Trail just south and east of Yellow Rose Spring (R23; 10 on 17 May 1998, L. J. Hargrove), and on the ridge 1.0–1.4 miles northeast of Buckman Springs (Q22; nine singing males 25 April 1999 and 11, including eight singing males, 10 June 1999, J. K. Wilson). Outward from this hub, the population becomes quickly more scattered but extends northwest to near Tule Springs (N18; four, three singing males and one juvenile, 2 July 2001, J. R. Barth et al.), west to 2.1 miles west of Corte Madera Mountain (R19; two singing males 8 June 2001, L. J. Hargrove), south to 4.7 miles east of Cameron Corners (U24; pair on 20 May 2000, A. Mauro, P. K. Nelson), and east to Sacotone Spring (S27; one singing male 17 May 2001, J. O. Zimmer, E. C. Hall). The population in San Diego County does not appear continuous with that in the Sierra Juárez south of the international border.
Between the two main populations we recorded the Gray Vireo on only five occasions in four atlas squares, with never more than two territorial males, on or near the east slope of the mountains in chamise-dominated chaparral. At one of these locations, however, the Pacific Crest Trail south of Barrel Spring (G20/H20), A. G. Morley had counted eight on 10 June 1987. Outliers in the south were one singing male 0.4 mile west of El Capitan Dam (O16) 21 April 1998 (K. J. Winter) and another in juniper woodland along the Imperial County line 4.4 miles east of Jacumba (U29) 27 April 1999 (J. K. Wilson). The latter was the only record during the atlas period from juniper woodland away from chaparral, though six were reported from such habitat 1972–84 (Anza–Borrego Desert State Park records). At another outlying site, 2500 feet elevation on the southwest slope of Potrero Peak (U19), there was a singing male 2 June 1992 (P. Unitt) but none could be found 1997–2001. At such locations away from the main population centers the Gray Vireo appears to be irregular. The bird near El Capitan Dam was at the unusually low elevation of 640 feet. Otherwise the sites range in elevation from 2100–2300 feet along Goudie Road to 5400 feet east of La Posta Creek in the Cuyapaipe Indian Reservation (one singing male 11 May 2001, D. C. Seals).
Although vast tracts of seemingly suitable and uniform chaparral remain, the Gray Vireo’s distribution is distinctly clumped in only a small fraction of this habitat, even within the two main zones of concentration. For example, five singing males along Goudie Road (O18) 6 June 1999 (L. J. Hargrove) were isolated by at least 8 miles from their nearest known neighbors to the south and east in the direction of the population center. Much of the Gray Vireo’s range consists of rugged hills, covered with impenetrable chaparral, in which unknown numbers may remain out of earshot. But enough roads and trails crossing the habitat have now been surveyed to demonstrate the species’ rarity and pattern of dispersion. The county’s population is probably in the low hundreds—small, but still larger than any known elsewhere in California.
Nesting: The Gray Vireo builds its nest in the upper levels of the shrubs that constitute its habitat. Nests described by atlas observers were 3 to 5 feet off the ground in chamise, scrub oak, cupleaf ceanothus, and mountain mahogany. Because of these shrubs’ intricately branched structure, the Gray Vireo’s nest may be more extensively supported from the sides than the typical vireo nest hanging from a horizontal fork. Data on the nesting schedule of the Gray Vireo are still meager; no collected eggs are known from San Diego County. Atlas participants’ observations suggest the species lays from late April to mid June.
Migration: The Gray Vireo arrives in San Diego County regularly in late March, rarely as early as 14 March (2001, one on the Pacific Crest Trail near Kitchen Creek Road, R23, L. J. Hargrove). It remains at least as late as 19 August (2001, five, three still singing, at the same locality, L. J. Hargrove) but may be difficult to find after the birds stop singing and the chaparral is hot and still. In the San Jacinto Mountains, the Gray Vireo is known as late as 27 August (Grinnell and Swarth 1913), in Joshua Tree National Park as late as 10 September (Miller and Stebbins 1964). There are five published records of migrants from the Anza–Borrego Desert (AFN 13:456, 1959; AB 42:482, 1988; Massey 1998), plus records of two at Bonita (T11) 1 May 1962 (AFN 16:448, 1962) and one at Palomar Mountain (D15) 19 September 1981 (AB 36:219, 1982). The Gray Vireo is seen in California as a migrant away from breeding habitat so rarely, however, that most individuals evidently commute between the breeding and winter ranges in a single nonstop flight. Some of the reports of migrant Gray Vireos may represent misidentified Bell’s or Plumbeous Vireos.
Winter: Wintering of the Gray Vireo in the elephant trees of the Anza–Borrego Desert was one of the most notable discoveries emerging from field work for this atlas. Previously the species was not known to winter in California. Because the fruit of the elephant tree is a principal food of Gray Vireos wintering in Sonora (Bates 1992), we organized an expedition 4–5 December 1999 to search California’s largest stand of this plant, along Alma Wash west to Starfish Cove (K28/K29), for Gray Vireos. The expedition revealed a minimum of five individuals, all near fruiting elephant trees (Unitt 2000). On subsequent visits to this area, Lori Hargrove has found the Gray Vireo as early as 21 October but no more than a single bird per day and none in 2000–01. The quantity of elephant tree fruit varies and, apparently, the number of wintering Gray Vireos with it.
Conservation: A century ago, the Gray Vireo may have been more widespread and numerous than currently. Stephens (1878) reported the species “not uncommon” around Campo and reported or collected it at Julian (K20) and Oak Grove (C16) as well. After 1908, however, the species lapsed into obscurity, going unreported until Michael U. Evans rediscovered the county’s southern population in 1978. The population may have declined even since then, since the numbers along Kitchen Creek Road (Q23/R23) and La Posta Truck Trail (Q24/R24) 1997–2001 were less than in the late 1970s.
Cowbird parasitism has been inferred as the likely cause of the Gray Vireo’s rarity in southern California (Remsen 1978), though the species has always been sparse; Grinnell and Swarth (1913) estimated 16 pairs per square mile in the San Jacinto Mountains. Friedmann (1963) considered the Gray Vireo a frequent victim of the Brown-headed Cowbird. The degree to which cowbirds are currently affecting the Gray Vireo is not known but needs study before any management to benefit the vireo is undertaken.
Because the Gray Vireo is so localized, it is susceptible to fire. Much of the area now occupied by San Diego County’s southern population was burned in the Laguna fire of October 1970. The Gray Vireo is likely disfavored both by frequent fire that keeps chaparral low and open and by fire suppression that leads to fuel buildup and catastrophic large-scale fires. The species’ ecology with respect to postfire succession needs study too.
Yellow-throated Vireo Vireo flavifrons
The Yellow-throated Vireo is a rare vagrant from the eastern United States, more frequent in California in spring than in fall. San Diego County seems to have got less than its fair share; though there have been at least 90 well-supported records in California, 1963–2003, only six are from San Diego County.
Migration: Two of San Diego County’s Yellow-throated Vireos have occurred in fall, both in Fort Rosecrans National Cemetery, Point Loma (S7): one photographed 13–20 November 1985 (M. and D. Hastings, Bevier 1990), another 19 October 2002 (P. A. Ginsburg, Cole and McCaskie 2004). One in spring was also at Point Loma, 17 May 1992 (G. L. Rogers, Heindel and Patten 1996). One was singing from willows and coast live oaks along old Highway 80 in Pine Valley (P21) 11–16 June 2001 (M. B. Mulrooney; Garrett and Wilson 2003), and another was along the San Diego River just below Old Mission Dam (P11) 20–21 June 2001 (T. Pepper, G. McCaskie).
Winter: One in Greenwood Cemetery (S10) 11 December 2003–7 January 2004 (J. O. Zimmer) was only the third Yellow-throated Vireo reported wintering in California.
Plumbeous Vireo Vireo plumbeus
In sharp contrast to most vireos, whose ranges have shrunk, the Plumbeous Vireo has spread. It was first noted in southern California in 1962, small numbers colonizing the east slope of the Sierra Nevada, San Gabriel Mountains, and San Bernardino Mountains, and becoming a regular and widespread if still rare migrant and winter visitor.
Migration: In San Diego County, the Plumbeous Vireo has been recorded from 16 September (1973, one in the Tijuana River valley, G. McCaskie) to 10 May (1981, one on Palomar Mountain, D15, R. Higson). From 1997 to 2001, 12 spring migrants were reported, between 26 March (2000, one at Yaqui Well, I24, E. Moree) and 24 April (2001, one at Point Loma, S7, J. C. Worley; one in Wonderstone Canyon, D29, R. Thériault). Spring migrant Plumbeous Vireos are scattered throughout San Diego County; the 12 during the atlas period were evenly split between the coastal lowland and the Anza–Borrego Desert. All records were of single individuals except for two in oak woodland in Poway 1.9 miles west-south of Starvation Mountain (K12) 16 April 1998 (E. C. Hall).
A unique summer record is of one at Banner (K21) 26 June 1994 (P. D. Jorgensen). A few Plumbeous Vireos have colonized the San Bernardino Mountains, a considerable jump in the breeding range, but the breeding habitat is pine woodland, not the riparian woodland found at Banner.
The first specimen of the Plumbeous Vireo for San Diego County was an immature found dead on at Mission Santa Ysabel (I18) 1 November 2002 (A. Mercieca, SDNHM 50893).
Winter: The Plumbeous Vireo is a regular winter visitor in the coastal lowland, mainly in riparian woodland, occasionally in urban trees. From 1997 to 2002, between 17 and 26 individuals were reported, depending on how many birds seen in the same area in successive winters were returnees. All records were of single individuals except for three at the Dairy Mart pond in the Tijuana River valley (V11) 19 December 1998 (G. McCaskie). The only wintering Plumbeous Vireos outside the coastal lowland during the atlas period were from planted pines in Borrego Springs (F24), 1 November–20 December 1998 (P. D. Jorgensen, R. Thériault) and 17 December 2000 (N. Osborn). Two earlier winter desert records are of one at Butterfield Ranch (M23) 8 February 1987 (D. B. King, AB 41:331, 1987) and one at Yaqui Well 11 February 1989 (D. and M. Hastings, AB 43:367, 1989). The species is now annual in winter in the Salton Sink (Patten et al. 2003).
Conservation: The breeding range of the Plumbeous Vireo is spreading gradually north and west (Johnson 1994), and this change has been accompanied by an even more striking shift in the species’ distribution in migration and winter. In San Diego County the Plumbeous Vireo was first noted 16 November 1969, with one in the Tijuana River valley (G. McCaskie). Soon after that occurrences became annual. Numbers seen on San Diego County Christmas bird counts, however, have remained stable at about two per year since the mid 1980s.
Taxonomy: Plumbeous Vireos in the United States are all nominate V. p. plumbeus Coues, 1866—if the species is indeed polytypic. The genetic studies supporting the split of the Solitary Vireo into the Plumbeous, Cassin’s, and Blue-headed were confined to the United States (Johnson 1995), and the “Plumbeous Vireos” resident in southeastern Mexico and Central America look more like Cassin’s (Phillips 1991).
Cassin's Vireo Vireo cassinii
William E. Haas
Rare in San Diego County as a breeding bird, Cassin’s Vireo is best found by visiting oak and coniferous woodlands above 4000 foot elevation from late May to mid July. It is seen throughout the county in migration, though uncommonly. It is a regular winter visitor to riparian woodland and ornamental trees near the coast but rare in that role as well. Though Cassin’s Vireo was never abundant, parasitism by cowbirds is likely responsible for the vireo’s population decline and current rarity.
Breeding distribution: Currently, Cassin’s Vireos summer in San Diego County only in montane mixed oak and conifer woodland above 4000 feet elevation. On Palomar Mountain, our 15 records during the atlas period were all of single individuals except for one nesting pair near Doane Pond (E15) 12 June 1999 and two nesting pairs at the lower end of Jeff Valley (F16) 30 May 1999 (W. E. Haas). Around Hot Springs Mountain, our only record during the atlas period was of a nesting pair at the north base in a small pocket of incense cedars in Lost Valley (D20) 10 June 2000 (W. E. Haas), though previously the species occurred near the summit (E20/E21; five, including a nesting pair, 8 June 1985, P. Unitt). Similarly, on Volcan Mountain and around Julian during the atlas period we had only three observations of single individuals, though numbers not long before were higher; for example, five on Volcan (I20) 30–31 May 1993 (P. Unitt). In the Cuyamaca Mountains we had 13 sightings at various sites, the best of which was Middle Peak (M20), with up to four, including three singing males, 11 June 2000 (R. E. Webster) and nesting of two pairs confirmed 3 June 1998, 18 June 2000, and 9 June 2001 (W. E. Haas). In the Laguna Mountains the 11 observations were concentrated around Wooded Hill and the heads of Agua Dulce and La Posta creeks (P23), with up to five on 3 June 1999 (E. C. Hall, J. O. Zimmer). The only sighting in the breeding season away from these areas was of one about 3500 feet elevation 2.6 miles southeast of Mesa Grande (I17) 15 June 2000 (D. C. Seals).
Nesting: Like many vireos, the male Cassin’s is not hesitant to sing at or near the nest, which is typically suspended by its rims from a forked twig of a small forest tree or bush. Nests are rarely placed at heights of more than 20 feet; most are built between 3 and 7 feet of the ground. Of six San Diego County nests, one nest was in a tangle of snowberry, honeysuckle, and an unidentified oak (Palomar State Park), two were in coast live oaks (Jeff Valley), and three were in incense cedars (two on Middle Peak, one in Lost Valley). Although Cassin’s Vireo nests are typically shaded in dense foliage (Harrison 1978), as were those in incense cedars, one in Jeff Valley was in a sparsely foliaged oak directly over a major footpath.
Our few observations of Cassin’s Vireo nesting 1997–2001 suggest laying of a single clutch from late May to mid June: four records of nest building 30 May–10 June, five of occupied nests 3–12 June, one of a nest with nestlings (Middle Peak 18 June 2000, W. E. Haas), and one of fledglings (Laguna Mountains 18 June 2000, E. C. Hall, J. O. Zimmer). These sightings all fit within historical data on Cassin’s Vireo nesting in San Diego County, 40 egg sets collected 15 May–28 June 1895–1933.
Migration: Spring migration of Cassin’s Vireo through San Diego County is spread rather evenly from 10 April to 20 May. Our earliest migrants were single birds at Bonsall (E8) 21 March 1999 and Carlsbad (I7) 23 March 1999 (P. A. Ginsburg); latest were along Vallecito Creek near Campbell Grade (M23) 20 May 1999 (R. Thériault) and at Crestwood Ranch (R24) 24 May 1999 (D. C. Seals). The extreme early date of 14 March 1970 still stands (AFN 24:643, 1970). Early arrivals are probably of migrants headed farther north; birds breeding in San Diego County’s mountains arrive later. A survey of Jeff Valley 15–16 May 1999 revealed no Cassin’s Vireos (W. E. Haas).
In fall, Cassin’s Vireo migration, extending from early September to November, is even less noticeable than in spring; most birds probably keep to the mountains.
Winter: The usual winter habitat for Cassin’s Vireo is riparian woodland in the coastal lowland, as at Kit Carson Park (J11; one on 12 December 1998, W. Pray), lower Los Peñasquitos Canyon (N8; one on 1 March 1998, D. K. Adams), and the species most consistent site, around the Dairy Mart pond in the Tijuana River valley (V11; up to two on 26 December 1998, G. McCaskie). Cassin’s Vireo also winters in ornamental trees, as at Greenwood Cemetery (S10; 12–21 January 2002, G. McCaskie), rarely in coast live oaks, as Steltzer County Park (O14; 8 January 2002, M. B. Stowe). The two winter records for the Anza–Borrego Desert are of one at Agua Caliente Springs (M26) 10 February 1977 (J. L. Dunn) and one at Ellis Farms, Borrego Valley (F25), 15 February 2002 (P. D. Ache).
Through the atlas period 1997–2002, the number of wintering Cassin’s Vireos found annually ranged from one to five. Four on the Oceanside count 22 December 1979 is the record for a San Diego County Christmas bird count.
Conservation: Cassin’s Vireo was more widespread and common in the past than now. Egg collections up to 1933 attest to its nesting at somewhat lower elevations than currently, at Witch Creek (J18), Descanso (P19), Campo (U23), and even Lake Hodges (K11). Cooper (1874) called Cassin’s Vireo “not rare” in the Cuyamaca Mountains and, just as the cowbird was beginning its population explosion, Stephens (1919a) called it a "rather common summer resident of timbered cañons in the mountains." Presumably brood-parasitism by the Brown-headed Cowbird is the factor most responsible for the change; cowbirds parasitize Cassin’s Vireo more heavily than expected from its abundance (Goguen and Curson 2002). Yet in spite of widespread cowbird trapping in San Diego County since the late 1980s, Cassin’s Vireo, unlike Bell’s, has yet to show any evidence of recovery. From the viewpoint of San Diego County, the trend toward increase reported for the species as a whole (Sauer et al. 2003) looks unlikely; in all of the species’ local roles the trends look flat to negative.
Taxonomy: With the split of the Solitary Vireo into the Blue-headed, Plumbeous, and Cassin’s, the last consists of only two subspecies, nominate V. c. cassinii Xantus, 1858, being the only one in the United States.
Blue-headed Vireo Vireo solitarius
The Blue-headed Vireo breeds east of the Rocky Mountains and reaches California primarily as a fall vagrant. Its reclassification as a species distinct from the Cassin’s and Plumbeous Vireos has attracted birders’ attention to it, but it is at least as rare here as birds with a similar breeding range like the Philadelphia Vireo.
Migration: Of San Diego County’s 13 records of the Blue-headed Vireo, 12 are for fall, from 15 September (1991, Point Loma, S. Mlodinow, AB 46:151, 1991) to 10 November (Point Loma, AB 35:227, 1980). All fall records are from Point Loma except for three from the Tijuana River valley in the 1970s.
Winter: The single winter record is of one in San Diego 16 January–22 March 1987 (J. L. Dunn, AB 41:331, 1987). The Blue-headed Vireo winters at the latitude of San Diego in the southeastern United States, so further winter records here should be expected.
Taxonomy: Vireo s. solitarius (Wilson, 1810), smaller and greener backed than the Appalachian V. s. alticola, is undoubtedly the subspecies reaching California; the only specimen (San Nicolas Island, SDNHM 38562) is this. But the Blue-headed Vireo has not yet been collected or even photographed in San Diego County.
Hutton’s Vireo Vireo huttoni
A fairly common resident of San Diego County’s oak woodland, Hutton’s Vireo is the only nonmigratory member of its family in California. Its mindlessly monotonous song can be heard nearly year round. Beginning breeding early in the spring, before the Brown-headed Cowbird, Hutton’s Vireo was able to withstand this parasite’s invasion, which decimated San Diego County’s other breeding vireos. Hutton’s Vireo has even been able to spread into riparian woodland with few or no oaks and to colonize recently regenerated habitat.
Breeding distribution: As a characteristic bird of oak woodland, Hutton’s Vireo has a distribution largely following that of the coast live oak. It is most common in oak-dominated woodlands in the foothills, with daily counts of up to 15 in Boden Canyon (I14) 6 June 2000 (R. L. Barber), near Warner Springs (F19) 14 May 1999 (C. G. Edwards), and near Campo (U24) 9 May 1998 (C. R. Mahrdt). But it can be common too in riparian woodland in which oaks are a minority of the trees (16 along the Santa Margarita River north of Fallbrook, C8, 24 May 2001, K. L. Weaver). Furthermore, the vireo approaches the coast more closely in many places, following strips of riparian woodland with few or no oaks, though it is uncommon in such woodland. A few Hutton’s Vireos inhabit Balboa Park (R9), with records including an adult building a nest 2 August 1977 (D. Herron), one feeding a fledgling at the west end of the Cabrillo Bridge 7 June 2000 (P. A. Ginsburg), a fledgling collected 4 August 1991 (SDNHM 47660), and an adult collected 7 August 1928 (SDNHM 12160). Hutton’s Vireo now occurs even in southwesternmost San Diego County, along the Sweetwater River in Bonita (T11; e.g., pair with fledglings 22 May 1997, P. Famolaro) and in the Tijuana River valley (e.g., two singing males along the river east of Hollister St., W11, 19 June and 13 July 1999, P. Unitt).
Hutton’s Vireo can be common in lower montane forest mixed with conifers (13 near Pine Hills Fire Station, L19, 24 May 1988, P. Unitt), but it is uncommon between 4500 and 5500 feet elevation and absent from the summits of the county’s highest mountains. The eastern limit of the range largely coincides with the eastern limit of coast live oaks, but Hutton’s Vireo also extends in oakless riparian woodland down San Felipe Valley as far as Scissors Crossing (four, including a pair building a nest, near Paroli Spring, I21, 27 July 2000, J. O. Zimmer; up to three singing males west of Scissors Crossing, J22, 6 May 2002, J. R. Barth).
Nesting: Hutton’s Vireo builds a conventional vireo nest, attached to twigs by its rim and suspended from a fork. Three nests described by atlas observers were in the crowns of coast live oaks, as is typical for the species, but one was only 2 feet off the ground in a snowberry, exceptionally low. One of the oak nests was being built directly over busy Highland Valley Road (K13).
Late March to early July is the main season of Hutton’s Vireo nesting in San Diego County, but occasional birds start earlier, as illustrated by a pair building a nest near Old Mission Dam (P11) 2 March 1998 (K. J. Burns) and an active nest perhaps still under construction along the Sweetwater River near Highway 94 (R13) 12 March 1999 (J. R. Barth). Eggs have been collected elsewhere in California as early as 10 February, and nest building has been reported from San Diego County as early as the first week of February (Davis 1995). The nests under construction in San Felipe Valley 27 July 2000 and in Balboa Park 2 August 1977 were remarkably late. We saw no suggestion that birds laying after the first week of July actually fledged young.
Migration: Hutton’s Vireo is largely resident, but a few birds disperse out of their breeding habitat. The species is occasional on Point Loma (S7); records include one seen 16 October 2000 (V. P. Johnson) and specimens collected 7 January 1932 (SDNHM 15659) and 21 September 1987 (SDNHM 44823). The latter was a vagrant from northern California (see Taxonomy). The only record of a possible migrant in the Anza–Borrego Desert during the atlas period was of one near the mouth of Sentenac Canyon (I23) 17 April 1998 (P. K. Nelson), only 2 or 3 miles from the nearest point where the species is resident along San Felipe Creek. But there are earlier spring records, from Borrego Springs (G24) and Tamarisk Grove (I24), both 5 May 1991 (A. G. Morley), and one fall record, from Lower Willows (D23) 16 October 1994 (L. Clark, K. Smeltzer).
Winter: We saw only slight spread of Hutton’s Vireo away from breeding localities in winter. Most records of winter visitors at nonbreeding locations were of just a single individual. Numbers in the Tijuana River valley, though, were distinctly higher in winter than in summer, with up to seven in the northwest quadrant of the valley (V10) 16 December 2000 (W. E. Haas) and five in the southwest quadrant (W10) 15 December 2001 (G. L. Rogers). In the Anza–Borrego Desert Hutton’s Vireo is rare, and most records are from the desert’s edge. The only winter records from the desert floor are of one in north Borrego Valley (E24) 21 December 1997 (P. R. Pryde) and two at Tamarisk Grove and Yaqui Well 16 January 2000 (P. E. Lehman), with one still there 20 February (R. Thériault). Also of note are two in pinyon–juniper woodland in the Vallecito Mountains, one in Pinyon Mountain Valley (K25) 11 February 2000, the other on the east slope of Whale Peak (L26) 3 February 2000 (J. R. Barth).
Conservation: Unlike San Diego County’s other breeding vireos, Hutton’s did not suffer any great population decline after the invasion of the Brown-headed Cowbird. Though Hutton’s is a suitable and frequent host of the cowbird (we noted six instances of successful parasitism during observations for this atlas), some aspects of its biology buffer it from cowbird parasitism. Probably most important is that Hutton’s Vireo, as a resident species, often begins nesting well before breeding cowbirds arrive and start to lay, generally in late April. Furthermore, Hutton’s Vireo’s prime oak woodland habitat has been far less disturbed than the riparian woodlands favored by Bell’s Vireo. Hutton’s Vireo has, if anything, increased over San Diego County’s recorded history. Stephens (1919a) called the species only an “infrequent winter resident and occasional in summer.” Most of the species’ colonization of coastal riparian woodland outside the oak zone appears to have taken place since the early 1980s (cf. Unitt 1984). Such is certainly the case in the recently regenerated riparian woodland of the Tijuana River valley. This is the habitat largely responsible for increased numbers of Hutton’s Vireos on San Diego Christmas bird counts. From 1999 to 2001 the numbers ranged from 11 to 17; on no count before 1999 did the total exceed five.
Taxonomy: Bishop (1905) reported the Hutton’s Vireos of southern California to be darker and grayer than those of the central California coast, describing them as V. h. oberholseri and selecting a San Diego County specimen from Witch Creek (J18) as the type. Rea (in Phillips 1991) supported the recognition of oberholseri, though others had synonymized it with nominate huttoni. The specimen from Point Loma 21 September 1987 (SDNHM 44823) is conspicuously yellower than the local population and evidently a vagrant of V. h. parkesi Rea, 1991, which breeds along the coast of northern California from Humboldt to Marin County.
Warbling Vireo Vireo gilvus
The Warbling Vireo is a common migrant through San Diego County but rare as a breeding bird and very rare as a winter visitor. Migrants stop in any tree, but breeding birds seek mature riparian and oak woodland. Of any California bird, the Warbling Vireo is perhaps the most susceptible to cowbird parasitism. Cowbird trapping intended to benefit Bell’s Vireo is likely responsible for bringing the Warbling Vireo back—just barely—from the brink of extirpation as a breeding species in San Diego County. Atlas participants observed only two Warbling Vireo nests, but female cowbirds were scrutinizing or entering both of them while the vireos were building them and birders were watching them.
Breeding distribution: In San Diego County the breeding population of Warbling Vireos is now concentrated in the riparian woodlands in the county’s northwest, especially in the area of De Luz and Fallbrook. The largest number of breeding birds found, nine singing males 20 June 2000, was along De Luz and Cottonwood creeks north of the De Luz school (B6; K. L. Weaver). The Santa Margarita River near Fallbrook also supports several Warbling Vireo territories, with four singing males west of Sandia Creek Road (C7) 25 April 1998 and six east of that road (C8) 24 May 2001 (K. L. Weaver). Elsewhere in northwestern San Diego County, however, breeding Warbling Vireos are still uncommon to rare. At low elevations in southern San Diego County, the only area where the Warbling Vireo seems likely to breed is the Sweetwater River, with a singing male and a pair near the upper end of Sweetwater Reservoir (S13) 24 May 2000 (P. Famolaro) and one near Highway 94 (R13) 24 June 1998 (M. and D. Hastings).
A few breeding Warbling Vireos are also scattered through the county’s mountains. High numbers in this area are of six (all singing males) around Palomar Mountain (E15) 18 June 1999 (C. R. Mahrdt, E. C. Hall), nine (including three singing males and a courting pair) at Wynola (J19) 22 May 1999 (S. E. Smith), and four (including a pair) near Wooded Hill, Laguna Mountains (P23) 3 June 1999 (E. C. Hall, J. O. Zimmer). There is one summer record of the Warbling Vireo from riparian woodland at the desert’s edge: two, including a singing male, along San Felipe Creek 1.2 miles west northwest of Scissors Crossing 13 July 2001 (P. Unitt et al.). The species’ use of this site is evidently irregular; thorough surveys of the Scissors Crossing area in 2002 did not reveal any summering Warbling Vireos.
Determining the Warbling Vireo’s precise breeding distribution, however, is difficult. Spring migrants are still moving through late in the spring (see Migration), long after the local population has begun nesting. Ideally, any assessment of whether Warbling Vireos are territorial or paired would be followed up with observations after the last of the migrants are gone. This was not possible in every instance; a few squares where the species is shown as possibly breeding are based on observations of apparently territorial singing males during the spring migration period only. Nevertheless, none of these locations lies more than 8 miles from locations where the species occurred in midsummer.
Nesting: We confirmed nesting of the Warbling Vireo only five times from 1997 to 2001, on the basis of an adult feeding a fledgling near De Luz (B6) 22 July 2000 (K. L. Weaver), a fledgling along the Santa Margarita River northwest of Fallbrook (C7) 4 July 1998 (K. L. Weaver), a pair adding to a nearly completed nest at 1900 feet elevation in Castro Canyon (C12) 13 May 2000 (J. Determan, P. Unitt), a singing male with a fledgling at 4550 feet elevation 1 mile north of San Ignacio on the northeast flank of Hot Springs Mountain (E22) 21 June 2001 (J. R. Barth), and a bird building a nearly completed nest (that later failed) at Cibbets Flat (Q23) 29 May 1999 (A. Lazere, J. R. Barth). The nests in Castro Canyon and at Cibbets Flat were both in the outer canopy of coast live oaks. These sites are typical for the species, which commonly builds its nests in the middle to upper levels of tall trees. Dates of eight egg sets collected in San Diego County 1900–21 range from 13 May to 16 June, and none of the recent observations augments this interval.
Migration: In spring migration, the Warbling Vireo occurs throughout San Diego County, but, as with many other species, the largest numbers are seen along the east base of the mountains (25 at Vallecito, M25, 29 April 1997, M. C. Jorgensen; 23 at Agua Caliente Springs, M26, 12 May 1997, E. C. Hall). It begins arriving by late March, but occasional birds show up earlier. From 1997 to 2001, the first spring report varied from 27 February to 29 March. Occurrences before 15 March, though, are rare, with only six reported during the atlas period. The two sightings that coincided on 27 February 1999, of two in upper San Felipe Valley (H20; A. P. and T. E. Keenan) and one at Yaqui Well (I24; P. D. Jorgensen), are the earliest recorded in San Diego County. Spring migration peaks in late April and early May. The Warbling Vireo is not only an early migrant but a late one, seen regularly into the first week of June. The latest apparent migrant reported 1997–2001 was one in Tecolote Canyon (Q9) 15 June 2000 (T. Plunkett); previous records extend to 22 June (1977, Point Loma, S7, P. Unitt).
Fall migration extends from August through October at least, peaking in September. Nearly all Warbling Vireos passing through San Diego County in September and October are immatures.
Winter: The Warbling Vireo is rare in winter, occurring in urban trees in a narrow strip along the coast. Four were noted during the atlas period, at Buena Vista Lagoon (H5) 22 December 2001 (J. Determan, NAB 56:224, 2002), at the Marine Corps Recruit Depot (R8) 15 December 1998 (P. A. Ginsburg, NAB 53:210, 1999), at North Island Naval Air Station (S8) 20 December 1997 (R. T. Patton), and at Coronado (S9) 16 December 2000 (E. Copper). Most of the approximately 20 winter records of the Warbling Vireo are from metropolitan San Diego; only three are from the north county. Most are for December, in part an artifact of the scheduling of Christmas bird counts, but suggesting that most of these stragglers do not remain or survive the entire winter. There are only two records from San Diego after 10 January, of one that remained to 24 January 1984 (P. E. Lehman, AB 38:358, 1984) and another 11 February 1985 (R. E. Webster, AB 39:211, 1985).
Conservation: Early in the 20th century, the Warbling Vireo was considerably more common in San Diego County. Sharp (1907) said that, as a breeding bird, it was “not uncommon around upper end of Escondido Valley.” Stephens (1919a) called it a “common summer resident” in San Diego County’s mountains. Willett (1912) wrote that it was a “common summer resident, locally” on the coastal slope of southern California generally. Sites of eggs collected 1900–20 included Descanso (P19), Lakeside (O14), El Monte (O15), and Dulzura (T16), where the species is now absent. However tenuous these data may be, they suggest that the Warbling Vireo had declined precipitously by the late 1970s, when the breeding population was on the verge of extirpation from San Diego County (Unitt 1984). Parasitism by the Brown-headed Cowbird is almost certainly the major factor responsible for this decrease. The vireo’s decline followed the cowbird’s invasion about 1915. The success of parasitized nests is low to none (Rothstein et al. 1980). And, in the Sierra Nevada, vireo and cowbird abundances are related inversely (Verner and Ritter 1983). Low success of even unparasitized nests may make the Warbling Vireo unusually susceptible to the cowbird (Ward and Smith 2000). Evidence is mounting that the decline is spreading from San Diego County north into the core of the range of subspecies swainsonii (Gardali and Jaramillo 2001).
A minor rebound in the number of breeding Warbling Vireos in northwestern San Diego County, at least, has followed the widespread trapping of cowbirds since the mid 1980s. But the rebound is far less than that of the Blue-gray Gnatcatcher or Bell’s Vireo.
Taxonomy: Only the Pacific coast subspecies of the Warbling Vireo, V. g. swainsonii Baird, 1858, has been collected in California. Differences between swainsonii and nominate gilvus in molt schedule (Voelker and Rohwer 1998) and response to cowbird parasitism (Sealy et al. 2000) imply that the distinction between the eastern and western Warbling Vireos is more profound than the minor differences in size, plumage color, and bill shape suggest.
Philadelphia Vireo Vireo philadelphicus
Breeding in the boreal forest east of the Rocky Mountains, wintering mainly in southeastern Mexico and Central America, the Philadelphia Vireo is a vagrant to California. All but two of San Diego County’s 20 records are from a seven-week period in the fall.
Migration: Fall records of the Philadelphia Vireo in San Diego extend from 19 September (1975, Point Loma; 1976, Tijuana River valley) to 10 November (1999, Lindo Lake, Lakeside, O14, G. McCaskie, NAB 54:105, 2000). The last record, of a bird found first on 3 November, is the only one away from Point Loma and the Tijuana River valley. Two specimens have been collected, California’s first Philadelphia Vireo, in the Tijuana River valley 9 October 1965 (SDNHM 35511, McCaskie 1968b), and another at Point Loma 9 November 1969 (SDNHM 37390).
The single spring record for San Diego County is of one at Point Loma 29–31 May 1982 (N. B. Broadbrooks, Morlan 1985).
Winter: San Diego County’s one winter record of the Philadelphia Vireo, of one in Coronado (S9) 15–16 December 2001, was only the fourth for California (R. E. Webster, NAB 56:224, 2002). Even in mid December, though, it may have been a late fall vagrant.
Red-eyed Vireo Vireo olivaceus
The Red-eyed Vireo is one of the most abundant birds in the unbroken forests of eastern North America and breeds west to Washington and Oregon. But it migrates mainly east of the Rocky Mountains and is only a rare vagrant to California. In fall, occurrences in San Diego County average about 0.75 per year. There are also four spring records.
Migration: There are at least 30 fall records of the Red-eyed Vireo in San Diego County, most from Point Loma (S7), some from the Tijuana River valley, one from Otay Mesa (V13), and one from the home of L. M. Huey in the Logan Heights neighborhood of San Diego (S10), where he collected California’s first on 6 October 1914 (Huey 1915, SDNHM 33177). Their dates extend from 9 September (1970, Tijuana River valley, AB 25:110, 1971) to 3 November (1974, same locality, AB 29:123, 1975). The largest number reported in a single year was four in 1989 (AB 44:164, 1990).
The spring reports are of single birds at Kit Carson Park (K11) 17 May 1980 (K. L. Weaver, AB 34:921, 1980), Point Loma 28 May 1995 (R. E. Webster, NASFN 49:310, 1995), the Tijuana River valley 5 June 1979 (AB 33:806, 1979), and Coronado (S9) 14 June 1993 (E. Copper, AB 47:454, 1993).
Taxonomy: The North American population of the Red-eyed Vireo is generally considered a single subspecies, V. o. olivaceus (Linnaeus, 1766).
Yellow-green Vireo Vireo flavoviridis
Though the Yellow-green Vireo breeds almost entirely in Middle America, barely crossing the border along the Rio Grande in southernmost Texas, it is highly migratory, wintering in Amazonian Peru and Bolivia. In California it is a rare but now annual fall vagrant, mainly along the coast.
Migration: Sixteen Yellow-green Vireos have been accepted from San Diego County by the California Bird Records Committee, on dates ranging from 6 September (1996, Point Loma, D. W. Aguillard, McCaskie and San Miguel 1999) to 25 October (1976, Tijuana River valley, Luther et al. 1979). A few other reports have been rejected or not submitted. All accepted records are from Point Loma or the Tijuana River valley except for the two specimens, both found dead or injured in residential areas of San Diego: one on 7 October 1967 (SDNHM 36247), the other 16 September 1996 (SDNHM 50241).
Taxonomy: The relatively dull northwesternmost subspecies of the Yellow-green Vireo, V. f. hypoleucus van Rossem and Hachisuka, 1937, breeding in Sonora and Sinaloa, seems the most likely one to reach California. A. R. Phillips identified the 1967 specimen as this. But the 1996 specimen has the yellow on the underparts conspicuously darker and should be compared with the yellower V. f. flavoviridis (Cassin, 1851).